1. Technical Field of the Invention
The present invention relates generally to a gas sensor which may be installed in an exhaust system of an internal combustion engine for air-fuel ratio control, and more particularly to an improved structure of a gas sensor designed to ensure a desired mechanical strength of built-in porcelain insulators in withstanding applied physical loads.
2. Background Art
Japanese patent First Publication No. 2001-343355 discloses a conventional gas sensor 9, as illustrated in FIG. 17, which is used in burning control of an internal combustion engine. The gas sensor 9 includes a housing 90, a first porcelain insulator 91, a second porcelain insulator 92, and a sensor element 8. The first porcelain insulator 91 is retained in the housing 90. The sensor element 8 is inserted into the first porcelain insulator 91. The second porcelain insulator 92 is disposed in abutment with a base end surface 910 of the first porcelain insulator 91.
The first porcelain insulator 91 and the second porcelain insulator 92 are typically made of an insulating ceramic material. When burned or fired, the ceramic material is usually subjected to variation in local shrinkage due to a variation in density of the ceramic material occurring during production thereof or quantity of heat the ceramic material undergoes during the firing thereof, thus resulting in micro undulations on the base end surface 910 of the first porcelain insulator 91 and a top end surface 920 of the second porcelain insulator 92.
The undulations may result in undesirable shifting of a contact area 93 between the first porcelain insulator 91 and the second porcelain insulator 92 from an annular gasket 94 through which the first porcelain insulator 91 is seated on the housing 90 in a direction perpendicular to the length of the gas sensor 9. This causes the bending stress to act on the first porcelain insulator 91 which would lead to breakage of the first porcelain insulator 91.
In order to avoid the above problem, the second porcelain insulator 92 is aligned to the first porcelain insulator 91 so that the contact area 93 is located within a region T defined by projecting the profile of the annular gasket 94 disposed on an inner shoulder 900 of the housing 90 onto the base end surface 910 of the first porcelain insulator 91 in an axial direction of the gas sensor 9, thereby holding the contact area 93 from being shifted horizontally to minimize the bending stress which would result in the breakage of the first porcelain insulator 91.
The above structure, however, still has the problem in that within the contact area 93, the first and second porcelain insulators 91 and 92 are in abutment of curves surfaces of tops of the undulations to each other through which the first porcelain insulator 91 and the second porcelain insulator 92 receive the load from each other, which may result in the breakage thereof.
Usually, the ceramic material has many inherent defects such as pores or micro cracks in the surface thereof. Such defects, therefore, exist in the base end surface 910 of the first porcelain insulator 91 and the top end surface 920 of the second porcelain insulator 92, thus leading to a greater concern about the breakage occurring when the first and second porcelain insulators 91 and 92 are subjected to the physical load.